@artem thanks for your help. I removed the “Products navigation” option in Theme Settings -> Single product -> Elements and also did the same for the custom product layout, and then the smart Paypal buttons appear on my website. Now the question is why it has an issue with the product navigation element? What if I also want to display product navigation, because this element is very helpful for customer ease.

hey @chardbraceros, you can do the same maybe it also works for you. But yeah you may need some CSS code to fix the size of the buttons on the single product page.